Wireless transceivers are calibrated to compensate for errors introduced by the electronic signal processing components in the transceivers. During closed loop calibration, a signal having some known properties is injected into the transmitter. The transmit signal generated by the transmitter is input, or looped back, to the receiver. The “calibration signal” generated by the receiver in response to the looped back signal is used to determine errors introduced into the transceiver by the various transceiver components. Adjustments are made to components in the transceiver based on the determined errors.
For example, adjustments may be made to digital pre-distortion circuitry that pre-distorts the signal provided to the transmitter and post-distortion circuitry that post-distorts the signal output by the receiver. The pre-distortion circuitry includes components that adjust the gain and/or phase of the signal provided to the transmitter to compensate for the errors that will be introduced into the signal by the transmitter. The post-distortion circuitry includes components that adjust the gain and/or phase of the signal output by the receiver to compensate for the errors that were introduced into the signal by the receiver.
One type of error experienced by quadrature transceivers is IQ mismatch. The quadrature receiver downconverts a quadrature baseband signal from a radio frequency (RF) signal to a quadrature baseband signal. More specifically, the receiver includes a mixer that mixes the RF signal with an in-phase component of a local oscillator signal to downconvert the radio frequency signal to an in-phase component (i.e., the “I” signal) of the quadrature baseband signal. The mixer also mixes the RF signal with a quadrature component of the local oscillator signal to downconvert the radio frequency signal to a quadrature component (i.e., the “Q” signal) of the quadrature baseband signal. The I signal and the Q signal are processed on separate paths of components within the receiver before being recombined for demodulation.
However, the gain of the components in the I path and the Q path may not match exactly. As a result, a gain error is introduced into the quadrature baseband signal. In addition, the in-phase component and the quadrature component of the local oscillator signal used by the mixer for downconversion may not be exactly ninety degrees out-of-phase. As a result, a phase error, or quadrature error, is introduced into the quadrature baseband signal. Combined, the gain and quadrature errors form an I/Q mismatch error in the receiver. It is desirable to correct the I/Q mismatch of the receiver in baseband in order to provide improved performance.